The use of battery backup power for electronics devices in the event of primary power unavailability has been in use for decades. In general terms, an output voltage of a power section is required for operation of circuitry in equipment. This output voltage is normally derived from the primary power source in normal operation, which may be, for example, DC derived from an AC source. If the primary power source is for some reason unavailable, the output voltage is then derived from an internal battery source as part of a backup power mode. When primary power becomes available again operation is switched back to the primary power source. This switchover from primary power to battery power, and vice versa, is controlled by a supervisory circuit in the power section of the device.
The battery may be charged when primary power is available, and the device is operating in the normal operating mode. The amount of battery charging current must be controlled or limited, as overcharging may result in the battery overheating, becoming a fire hazard, and reducing battery life. The current UL (Underwriters Laboratory) recommendation for battery overcharging protection involves a resistor in series with the battery and the charging source. While this resistor will limit charging current into the battery, it will produce a voltage drop across the resistor during battery operation. This voltage drop is often in conflict with other requirements, such as minimal operating voltage requirements for device circuits that receive battery power. An excessive voltage drop may make following circuitry inoperable if its voltage requirements are not met.